Conventionally, a liquid crystal display element such as a liquid crystal display cell is fabricated by setting two electrode-bearing transparent substrates face to face at a prescribed distance, forming a cell by sealing their surrounding with a sealant, injecting a liquid crystal into the cell through a liquid crystal injection inlet formed in a portion of the cell, and sealing the liquid crystal injection inlet by the sealant or an end-sealing material.
In this method, at first, a seal pattern having a liquid crystal injection inlet is formed on one of the two electrode-bearing transparent substrates by screen printing using a thermosetting sealant and then the solvent in the sealant is dried by pre-baking at 60 to 100° C. Next, the two substrates are set face to face while sandwiching a spacer between them, aligned, stuck to each other, and thermally-pressed at 110 to 220° C. for 10 to 90 minutes, and after the gap adjustment in the vicinity of the sealant, the sealant is partly cured by heating at 110 to 220° C. for 10 to 120 minutes in an oven. After that, a liquid crystal is injected through the liquid crystal injection inlet and finally the liquid crystal injection inlet is sealed by an end-sealing material to fabricate a liquid crystal display element.
However, there are the following problems in this fabrication method: displacement due to thermal press, unevenness of gaps, and deterioration of adhesion strength between a sealant and a substrate take place: foaming occurs by the thermal expansion of a remaining solvent to result in unevenness of gaps and sealant pass: it takes a long time for sealant curing: the pre-baking process is complicated: the usable time of the sealant is short owing to solvent evaporation: injection of a liquid crystal takes a long time: and the like. Above all, in the case of a recent liquid crystal display apparatus with a large size, it becomes a serious problem that injection of a liquid crystal takes rather a long time.
For that, a liquid crystal display element fabrication method, so-called one drop fill process, using a photocuring and thermocuring sealant has been investigated. In the one drop fill process, a rectangular seal pattern is formed at first in one of two electrode-bearing transparent substrates by screen printing. Next, fine droplets of a liquid crystal in an uncured state of the sealant are dropwise applied to the entire face of the entire frame of the transparent substrate and immediately the other transparent substrate is layered and UV rays are radiated to the sealed part to carry out temporal curing. After that, at the time of liquid crystal annealing, the curing is actually carried out by heating to fabricate a liquid crystal display element. If the substrates are stuck to each other in reduced pressure, the liquid crystal display element can be fabricated at an extremely high efficiency. In the future, this one drop fill process is expected to be a main stream of methods for fabricating liquid crystal display apparatus.
Sealants employed in conventional processes are, for example, adhesives mainly containing partly (meth)acrylated bisphenol A type epoxy resins disclosed in Japanese Kokai Publication Hei-6-160872, 1-243029, 7-13173, 7-13174, 7-13175, and the like and also liquid crystal sealants mainly containing (meth)acrylates disclosed in Japanese Kokai Publication Hei-7-13174.
However, these sealants tend to exhibit polarity values similar to those of liquid crystal materials and both have affinity to each other. Accordingly, in a liquid crystal display element assembled using such a sealant by the one drop fill process, a component of the sealant may be eluted in the liquid crystal to cause orientation disorder of the liquid crystal in the peripheral part of the sealant and result in defective display such as uneven coloration. Especially, since the one drop fill process involves a step of bringing the uncured sealant into direct contact with the liquid crystal, the liquid crystal pollution with such a component of the sealant has become a serious problem. Further, among the above-mentioned sealants, a sealant comprising a thermosetting component has a problem that it is considerably inferior in storage stability at a room temperature and therefore requires to be stored in a frozen state and being contaminated with water at the time of thawing or once being thawed, it becomes thickened at a room temperature and therefore has to be used in a short time. Moreover, in the case of a sealant to be cured only by light radiation, it is poor in humidity resistance and a part shielded from light is inferior in the curing ratio.
Further, there are other problems relevant to residues such as an un-reacted polymerization initiator and a curing agent contained in a sealant after curing; an ionic impurity such as chlorine or the like; and a silane coupling agent. Owing to applications of liquid crystal panels to mobile appliances with saved power consumption, a liquid crystal with low driving voltage (a low voltage type liquid crystal) tends to be used in these years. The low voltage type liquid crystal is easy to take in impurities attributed to a particularly high dielectric anisotropy and easy to cause orientation disorder and decrease of voltage holding ratio with time. That is, owing to intake of the residues such as an unreacted polymerization initiator and an initiator after curing contained in a sealant; an ionic impurity such as chlorine; and a silane coupling agent, problems such as orientation disorder and decrease of voltage holding ratio with time are caused.
For that, countermeasures such as decreasing the amount of a polymerization initiator contained in the sealant or making a polymerization initiator into a high molecular weight has been investigated. However, such a method is not only incapable of sufficiently suppressing the elution to the liquid crystal but also decreases the reactivity, resulting in requirement of a large quantity of light rays for curing the sealant or the like and adverse effects on the liquid crystal.
Further, with respect to ionic impurities, Japanese Patent Kokai Publication Hei-5-295087 discloses a method for removing ionic impurities by washing a sealant, an end-sealing material, and their raw materials with water or an organic solvent and carrying out reduced-pressure drying. However, such a method requires extremely complicated process and in addition, the method sometimes causes such problems that if the drying of the washed sealant or end-sealing material is insufficient, the solvent remains or that gelling occurs at the time of reducing the pressure in the drying step.
Curing resin compositions to be used for a sealant or an end-sealing material can be broadly classified into two-liquid type that is used by mixing a main agent and a curing agent and one-liquid type that contains a curing agent previously. Although the two-liquid type one can be cured at a room temperature, a curing resin (a main agent) and a curing agent should be separately stored and in the use, it is required to measure and mix the respective components and thus its storage and handling is complicated. Further, since the pot life is limited, a large quantity of the respective components cannot be mixed previously and frequent mixing is required to result in a low efficiency. On the other hand, with respect to the one-liquid type one, reaction occurs during the storage in some cases and it is desired to improve the storage stability. For that, in the case where an epoxy resin is used for the curing resin, an one-liquid type curing resin composition using a dicyandiamide as a curing agent has been known well. However, the resin composition requires a high temperature of not less than 160° C., for curing and cannot meet the requirement, “low temperature and short time curing”, which has been required in recent years. As systems capable of satisfying “low temperature and short time curing”, there are methods proposed as follows: Japanese Kokai Publication Sho-62-146915 proposes a method using an adduct of an amine and an epoxy compound as a curing agent: Japanese Kokai Publication Hei-2-292325 proposes a method using an imidazole encapsulated by poly(methyl methacrylate) as a curing agent: and Japanese Kokai Publication Sho-59-59720 proposes a method for making the surface of a powder amine compound inert by an isocyanate. However, these methods are very complicated and have a problem of considerable elution of un-reacted amines or the like to liquid crystals.